The database landscape has been significantly diversified during
the last decade, resulting in the emergence of a variety of
non-relational (also called NoSQL) databases, e.g., XML and
JSON-document databases, key-value stores, and graph databases. To
facilitate access to such databases and to enable data integration
of non-relational data sources, we generalize the well-known
ontology-based data access (OBDA) framework so as to allow for
querying arbitrary databases through a mediating ontology. We
instantiate this framework to MongoDB, a popular JSON-document
database, and implement an prototype extension of the virtual OBDA
system Ontop for answering SPARQL queries over MongoDB.